I have only tried this at my NVidia Geforce4 MX 440 graphics card.. I don't know how to accelerate ATI cards... Heres the instruction:
1)Open RivaTuner
2)On the Target Adapter Window, Click Customize
3)Then check "Enable low-level hardware overclocking"
4)Then move the two sliders to anywhere at the middle of the second and third arrow
Note: Don't adjust it too high or your pc will end up hanging..
5)Then click Test
6)After the loading, Click OK..
7)Restart your PC
If you can't understand the step-by-step procedure, you can refer to this image..
Hope this helps!